Many types of dental cements are currently on the market, and have very extensive applications. Typical examples of such cements are zinc phosphate cements obtained by the reaction of zinc oxide with phosphoric acid, carboxylate cements obtained by the reaction of zinc oxide with polycarboxylic acids, and glass ionomer cements obtained by the reaction of aluminosilicate glass with polycarboxylic acids. Other temporary filling and sealing cements are also available. Their primary use is cementing of crowns, inlays, bridges and orthodontic appliance, lining of various cavities and filling such cavity as class I, class III, or class V cavity. In general, these dental cements make use of chemical reactions between acids and bases. In most cases, the acidic components are supplied in the form of an aqueous solution due to their much solubility in water. In some cases, they may be powdered partly or wholly and blended in a cement powder. The basic components are ordinarily by far more difficult to dissolve in water than are the acidic components and, in most cases, are supplied in the powdery form. For use, the dental cements are mixed together to effect reaction between the acidic components and the basic components. Immediately upon mixing, the cement mixtures show considerably strong acidity, and usually change to neutrality with the lapse of time. Thus, the neutralization reaction of the cement mixtures are not yet completed in the initial stage where they are applied in the mouth of patients. To put it in another way, an appreciable amount of the acidic components remains in the cement mixtures. For this reason, when the cement mixture is close to the pulp of the tooth to be treated, the patient may suffer an unpleasant, or even acute, pain due to the strong irritating action of the remaining acids. In particular, the zinc phosphate, silicate and silicophosphate cements markedly hold their acidic irritating action owing to the use of an aqueous solution of phosphoric acid. Although the carboxylate or glass ionomer cements show relatively weaker acidic irritating action as compared with the zinc phosphate cements, yet that irritating action is by no means eliminated.
On the other hand, a major problem with the dental cements applied in the mouth is that they dissolve gradually in the saliva through many years. More specifically, when prosthetics such as crowns or inlays are cemented, a portion of cement filled in the gaps between the prosthesis and teeth dissolve with the lapse of time, resulting in the generating of secondary caries. When the dental cements are filled in the cavities, they start to dissolve from their surface with the resulting in deterioration of their appearance. For this reason, a reduction in the solubility of cement is an important problem to be solved.
In an effort of preparing a dental cement which is substantially free from any acidic irritating action and has a reduced solubility, it has unexpectedly been found that such a dental cement can be obtained by making use of a small amount of one or more of a tannic acid-protein combination, tannic acid-formaldehyde combination, acetyl tannic acid and a metal salt of tannic acid (hereinafter referred to as the tannic acid derivative(s)) which are difficult to dissolve in water. The dental cement according to the present invention is also found to be effective in relieving the pain during cementing.